This invention relates to a method of filtering blood for preparing a plasma or serum sample from whole blood.
The type or concentration of blood components, such as metabolites, proteins, lipids, electrolytes, enzymes, antigens, and antibodies, is measured, in general, using a plasma or serum sample obtained by centrifuging whole blood. However, centrifuging takes labor and time. Particularly, centrifuging is unsuitable for an urgent case of measuring a small number of samples promptly and in site inspection, because of requiring a centrifuge and electricity. Thereupon, it has been investigated to separate serum from whole blood by filtration.
Several filtration methods using glass fiber filter have been known wherein whole blood is charged into the, glass fiber put in a column from one side of the column, and pressurized or evacuated to obtain plasma or serum from the other side (Japanese Patent KOKOKU Nos. 44-14673, 5-52463, Japanese Patent KOKAI Nos. 2-208565, 4-208856).
However, practical filtration methods capable of obtaining an amount of plasma or serum from whole blood necessary for measuring by an automatic analyzer have not been developed except a part of items, such as blood sugar.
On the other hand, the inventors developed a blood filter unit composed of a filter holder and a syringe. The filter holder is composed of a holder body which contains filter material and a cap which is screwed on the holder body. The filter material consists of, e.g. two sheets of glass fiber filter, one sheet of cellulose filter and one sheet of polysulfone microporous membrane (FIG. 1 of EP 785430 A1).
Another blood filter unit composed of a holder body and a cap was also developed. The holder body consists of a plasma receiver located on the upper side and a filter chamber located on the underside. The filter material put in the filter chamber is composed of six sheets of glass fiber filter and one sheet of polysulfone microporous membrane (Example 1 of EP 785012A1).
Generally, in blood filtration, it is difficult to obtain a necessary amount of plasma or serum by surface filtration wherein blood cells do not enter the inside of filtering material, because flexible blood cells covet the surface of the filtering material in a short time. On thee other hand, in the filtration using glass fiber filter or the likes depth filtration occurs wherein blood cells are entangled in glass fiber with permeating into the glass fiber filter in the depth direction. In the case of the blood filtration using glass fiber filter or the like, leakage of blood cells occurs occasionally at early stage of the filtration. It is also a problem that breakage of blood cells, i.e. hemolysis, tends to occur during filtering.
A great problem in blood filtration is the breakage of blood cells, and several reports concerning the problem have been made. However, any filtration technique capable of measuring potassium ion accurately, which varies by a very small amount of hemolysis, has not been reported.
The measurement of potassium ion is one of the most important items in clinical assay, and potassium ion is frequently measured. The potassium ion concentration of plasma of healthy persons is about 4 meq/L, and an slippage of only about 0.3 meq/L makes exact diagnosis difficult. Accordingly, measuring accuracy required is very high. Since blood cells contain potassium ions in an amount about 40 times as much as plasma, in order to measure potassium ion concentration of plasma at the practical level in viewpoint of clinical diagnosys, it is necessary to prevent breakage of blood cells and escape of potassium ion through blood cell membrane.
It is possible to obtain a very small amount (several tens .mu.l) of plasma by blood filtration using microporous membrane or glass fiber filter. However, when several hundreds .mu.l plasma is separated as disclosed later, blood cells are deformed and broken upon trapped by the pore of filtering material. As a result, accurate measurement is impossible due to the increase of potassium ion concentration of plasma obtained by the filtration.